Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.
In an example arrangement, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In more quantitative terms, one arrangement that is currently being developed with the goal of producing up to about 100 W of EUV power at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 40,000-100,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 40-100 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position (i.e. with very small “jitter”) over relatively long periods of time.
For a typical LPP setup, target material droplets are generated and then travel within a vacuum chamber to an irradiation site where they are irradiated, e.g. by a focused laser beam.
One technique for generating droplets involves melting a target material, e.g., tin, and then forcing it under high pressure through a relative small diameter orifice, e.g. 0.5-30 μm. Under most conditions, naturally occurring instabilities, e.g. noise, in the stream exiting the orifice may cause the stream to break-up into droplets. In order to synchronize the droplets with optical pulses of the LPP drive laser, a repetitive disturbance with an amplitude exceeding that of the random noise may be applied to the continuous stream. By applying a disturbance at the same frequency (or its higher harmonics) as the repetition rate of the pulsed laser, the droplets can be synchronized with the laser pulses.
If the repetitive disturbance signal has a single frequency, a micro-droplet is produced for each period of the disturbance waveform. To cause multiple micro-droplets to coalesce together into a larger droplet, the disturbance signal may be modulated and may employ multiple characteristic frequencies. For example, the disturbance waveform may include a main carrier frequency and one or more modulation frequencies, which is/are typically smaller than the main carrier frequency. An example modulation frequency may be implemented using a harmonic of the carrier frequency (such as for example a third of the carrier frequency). The modulation frequency/frequencies causes different micro-droplets to depart the nozzle at different velocities, thereby causing them to coalesce after exiting the nozzle.
In an example, a plurality of micro-droplets, such as 60 micro-droplets, may coalesce together to form a larger main droplet. The stream of main droplets may then be irradiated by pulses from the main drive laser beam (which may involve one or more main pulses and optionally one or more pre-pulses for each main droplet) to create the aforementioned plasma.
If some of the micro-droplets do not coalesce into a larger droplet, the stream of droplets may include both the larger main droplets and some micro-droplets that failed to coalesce. The existence of the micro-droplets that failed to coalesce (so-called “satellite droplets”) in the droplet stream represents a non-optimal situation.
For one, the main droplets are optimally sized to generate the desired EUV radiation. The presence of satellite droplets, i.e., micro-droplets that failed to coalesce, means that one or more of the main droplets lack optimal size/mass/shape for optimal irradiation. Further, if the micro-droplets are irradiated, some of the laser energy that should be directed toward the main droplets is instead diverted to these undesirable satellite droplets, resulting in reduced system performance. Additionally, the irradiation of satellite droplets in the stream of main droplets creates unwanted plasma and may cause unintended instability in the droplet stream.
For these and other reasons, it is desirable to detect the presence of satellite droplets. The present invention relates to methods and apparatuses for such detection.